If your looking at package that is tickets and
free dining and that is best deal then cancel your other reservation.
OR yes you can add park tickets and dining to room only reservation NOW they will cancel the first room only reservation and refund that and then rebook the new package and you will pay deposit for that.
However you can only have ONE discount at a time so if you get free dining can not use PIN discount UNLESS that pin came with the free dining package and apparently it did not or you would not be wanting to change it.
When calling reservations and you do not like first answer CALL!! CALL!! again. Have been there over 70 times and I book and change a lot of things. I love WDW but it is my money and I do what I have to until I get what we need for the best price. More money to drop in the parks then